Autoantibodies in thymoma-associated myasthenia gravis and their clinical significance

Myasthenia gravis (MG) is characterized by the development of antibodies that act against the acetylcholine receptor (AChR) present at the postsynaptic site of neuromuscular junctions. Some MG patients have antibodies that bind in a cross-striational pattern to skeletal and heart muscle tissue sections (striational antibodies). These antibodies react with the epitopes on the muscle protein titin, ryanodine receptor (RyR), and voltage-gated K channel α subunit, Kv1.4. Since these 3 molecules are expressed in the thymoma tissue of MG patients, striational antibodies are frequently detected in thymoma-associated MG. More severe MG symptoms in thymoma-associated MG may be due to the presence of striational autoantibodies. The anti-titin antibody, usually detected by enzyme-linked immunosorbent assay (ELISA), is found in 20-40% of all MG patients, and is more common in late-onset MG patients. The anti-RyR antibody, detected by Western blotting or ELISA, is found in 13-38% of all MG patients, and is known to inhibit Ca²⁺ release from sarcoplasmic reticulum and excitation-contraction coupling of the muscle. The anti-Kv1.4 antibody, detected by the immunoprecipitation assay with 35S-labeled extract from rhabdomyosarcoma cells, is found in 12-15% of all MG patients. Autoimmune myocarditis may develop in MG patients who have the anti-Kv1.4 antibody. In addition, the anti-Kv1.4 antibody is a useful marker to check the response to calcineurin inhibitors. In summary, the detection of striational antibodies provides more specific clinical information in MG patients.